For pattern dimension management in semiconductor manufacturing processes, a critical dimension scanning electron microscope (SEM) is widely used, in which an SEM is specialized only to semiconductors. FIG. 2A is the basic configuration of a previously existing critical dimension SEM. A primary electron beam 102 emitted from an electron gun 101 is narrowly focused at a capacitor lens 103, and two-dimensionally scanned over a sample 107 by a deflector 104. Typically, a relatively low accelerating voltage of about one kV is used for an accelerating voltage. Secondary electrons 120 generated from the sample 107 by applying the electron beam are captured at a detector 121, and thus a secondary electron beam image is obtained. On the secondary electron beam image, pattern edges are bright on the image due to a tilt angle effect or edge effect. Thus, the locations of the edges are detected by image processing methods to determine dimensions.
Reductions in the costs of semiconductor devices are achieved by decreasing chip areas by downscaling. However, increases in manufacturing costs such as lithography cancel the merits of the costs obtained by decreasing chip areas. In NAND flash memories, which are new schemes for cost reductions, the development of a technique (3D-NAND) is accelerating, in which memory cell arrays are stacked to form a three-dimensional memory cell array.
3D-NAND is formed through process steps in which after an electrode film and an insulating film are alternately stacked, a hole penetrated from a topmost layer to a lowermost layer is opened at one time (see FIG. 3A), a memory film is formed on the side surface of the hole, and then a columnar electrode is buried. The process steps of opening a hole determine the success or failure of this process. The key point is to provide a hole that is penetrated to the lowermost layer in proper diameter. Requests are to manage whether a hole is opened or not or manage the ratio of the top diameter to the bottom diameter of a hole.
For a technique of observing whether a hole is opened or not or observing the ratio of the top diameter to the bottom diameter of a hole, Patent Literature 1, for example, describes a scanning electron microscope. The scanning electron microscope provides high energy primary electrons with energy enough to cause the primary electrons to reflect off the side wall or bottom face of a groove or hole of a sample and penetrate the inside of the sample for escaping from the surface of the sample or for generating tertiary electrons on the surface of the sample. The scanning electron microscope applies these primary electrons to the sample for observing a hole pattern having an aspect ratio of around three. Patent Literature 1 shows exemplary accelerating voltages of 100 kV and 200 kV for primary electrons.
Patent Literature 1 describes a configuration in which reflected electrons are disposed between an objective lens and a sample and detected by a scintillator, and tertiary electrons having passed through the center hollow portion of the objective lens are extracted using an extraction electric field and detected by the scintillator.
On the other hand, in Patent Literature 2, an electron beam accelerated at a voltage of 50 kV or more is applied to a sample using a scanning electron microscope, and secondary electrons or tertiary electrons generated from the sample are detected by a scintillator for observing the inside of a hole or groove. Similarly to Patent Literature 1, a configuration is described in which reflected electrons are disposed between an objective lens and a sample and are detected by a scintillator, and tertiary electrons having passed through the center hollow portion of the objective lens are extracted using an extraction electric field, and detected by the scintillator.